(a) Field of the Invention
The present invention relates to a method for preparing polyarylene sulfide (hereinafter, referred to as “PAS”) and, more particularly, to a method for preparing PAS that involves adding a polymerization inhibitor to reactants to reduce the iodine content of the PAS and to enhance thermal stability.
(b) Description of the Related Art
As a typical engineering plastic, PAS has recently been in considerable demand as a material for high temperature and corrosive environments and electronics products due to its excellence in thermal resistance, chemical resistance, flame resistance, and electrical insulation properties. PAS is primarily used for computer parts, automotive components, protective coatings against corrosive chemicals, industrial chemical resistant fabrics, and so forth.
The only PAS that is commercially available is polyphenylene sulfide (hereinafter, referred to as “PPS”). The current industrial synthesis process for PPS involves a reaction of p-dichlorobenzene (hereinafter, referred to as “pDCB”) and sodium sulfide in a polar organic solvent such as N-methyl pyrrolidone. This process is known as “Macallum process”, which is disclosed in U.S. Pat. Nos. 2,513,188 and 2,583,941. Among several polar solvents suggested in the prior art, the mostly used one is N-methyl pyrrolidone. The process uses dichloro aromatic compounds as a reactant and yields sodium chloride as a byproduct.
The PPS produced in the Macallum process has a molecular weight around 10,000 to 40,000 and a low melt viscosity less than 3,000 poise. For higher melt viscosity, PPS is usually subjected to the curing process including application of heat below the melting temperature Tm and exposure to oxygen. During the curing process, the melt viscosity of PPS can be raised to a level required for general PPS uses through reactions such as oxidation, cross-bonding, or polymer chain extension.
The PPS obtained in the conventional Macallum process, however, has some fundamental disadvantages as follows.
Firstly, the use of sodium sulfide as a supply of sulfur needed in the polymerization reaction leads to a large amount of metal salt such as sodium chloride as a byproduct in the polymer product. The residual metal salt is contained in the polymer product to several thousand ppm even after the polymer product is washed out, not only to raise the electrical conductivity of the polymer but also to cause corrosion of the processing machinery and problems during spinning of the polymer into fibers. From the standpoint of the manufacturer, the use of sodium sulfide as an ingredient material ends up with the 52%-yield of sodium chloride as a byproduct with respect to the weight of the added material, and the byproduct, sodium chloride, is not economical but wasteful even when recycled.
Secondly, the properties of the polymer product are adversely affected during the curing process. For example, oxygen-driven oxidation and cross-bonding reactions turn the polymer product darker with more brittleness in the aspect of mechanical property.
Thirdly, like all the polymer products of solution polymerization, the final PPS product is prepared in a very fine powder form that relatively lowers the apparent density, causing inconvenience in carrying and some problems during the processing of the PPS into desired goods.
Beside the Macallum process, some other processes have been proposed in U.S. Pat. Nos. 4,746,758 and 4,786,713 and other related patents. These patents suggest that PAS can be prepared by directly heating diiodo compounds and solid sulfur rather than dichloro compounds and metal sulfide used in the existing process without using any polar solvent. This method consists of two steps, iodization and polymerization: the iodization step involves a reaction of aryl compounds and iodine to form a diiodo compound, and the polymerization step includes a reaction of the diiodo compound and solid sulfur to yield PAS having a high molecular weight. During the reaction, there occurs production of iodine in vapor form, which iodine is collectible and reused to react with the aryl compounds again. Hence, the iodine substantially acts like a catalyst in the reaction.
This method can solve the problems with the conventional processes. First, iodine yielded as a byproduct does not raise the electrical conductivity of the PAS product as metal salts usually do, and can be easily collected from the reactants to readily make its content in the final product lower than the content of metal salts in the conventional processes. The collected iodine is reusable in the iodization step, reducing the quantity of waste almost to zero. Second, the polymerization step using no solvent provides the PAS product in the pellet form like the conventional polyester product, avoiding a problem with the product in the powder form according to the prior art. Finally, this method raises the molecular weight of the final PAS product far more than the conventional processes and thus eliminates a need for the curing process that leads to inferior properties of the product.
This process however has two main disadvantages. First, the residual iodine in the molecular state is so corrosive to adversely affect the processing machinery when it is contained in the final PAS product even in a minute amount. Second, the use of solid sulfur in the polymerization step causes introduction of disulfide bonds in the final PAS product to deteriorate the thermal properties of the product including melting temperature.
Consequently, there is a need for studies to develop a method for effectively preparing a PAS that not only considerably lowers the content of iodine causing corrosion of machinery but also provides excellent properties such as thermal resistance, chemical resistance, and mechanical strength.